Sounders for use in fire alarm systems fall into two categories. The first category comprises standalone sounders as exemplified, for example, by the sounder of U.S. Pat. No. 6,905,001. In this form the sounder can be fitted to a ceiling or a wall. The second category comprises “behind the detector” sounders. In this form a base is fitted to the ceiling, the sounder is fitted to the base, and then the detector is fitted to the sounder. It is an essential requirement that the dimension of the stack of base, sounder and detector, measured vertically, be kept as small as possible. The diameter of the sounder has to be compatible with that of the base and detector.
This creates difficulties in designing a sounder which will produce not only the requisite volume of sound but also a sound distribution pattern which meets the demands of the specifications which the authorities in most industrialised countries have established. Another factor which complicates the construction of sounders of this type is that the power available to vibrate the diaphragm which produces the initial sound waves is low.
One of the components of a known “behind the detector” sounder is illustrated in the Figure designated “prior art”.
The sounder component is designated 1 and comprises a base wall 2 and a skirt 3. the sound paths, which extend radially, are designated 4 and are bounded by walls 5 which protrude upwardly from the base wall 1. The throats 6 of the sound paths are at the centre of the base wall 2 and their mouths 7 are at the periphery of the base wall 2. The spaces 8 within adjacent walls 5 receive the electrical components which connect the ceiling mounted base (not shown) to the electronics of the “behind the detector” sounder. The base closes the sound paths 4.
Columns 9 are provided for mechanically connecting the sounder component to the base.
The parameters which have to be taken into consideration in designing a horn are throat area, mouth area, the rate at which the horn flares from throat to mouth and the length of the horn from throat to mouth.
The freedom of the designer to vary the first three parameters is limited by various factors such as physical size, the cut-off frequency and the optimal load impedance. The fact that speech must be transmitted also increases the difficulties involved in designing a “behind the detector” sounder.
This leaves the designer with the length of the horn as the dimension which he can most readily vary to obtain a sounder having the desired characteristics.
The present invention seeks to provide a “behind the detector” sounder which has acceptable dimensions and which also provides sound of sufficient volume and with a distribution pattern that meets the applicable industry standards of the major industrial countries.